The present invention generally relates to a magnetron device and more particularly, to an improved magnetron to be incorporated in a high frequency microwave oven i.e. so-called electronic range or the like (referred to as an electronic range hereinafter).
In the field of electronic ranges which are coming into wide use in recent years, technical improvements have been continuously made in order to achieve the targets and satisfy the requirements both in the demand and supply with respect to compact size, light weight and reduction in cost, and under the above circumstances, reduction in size and weight, high operating efficiency, low noise and reduction of cost, etc. are also required for the magnetron to be incorporated in such electronic range.
With respect to the compact size and light weight which are of the first subject, improvement of a magnetic circuit is first brought into consideration.
In the magnetron commonly used, a ferrite magnet has been generally employed, and it was considered to replace the material for the magnet by Alnico (name used in trade and manufactured by General Electric Co., U.S.A.) or a samarium-cobalt (Sm-Co)alloy for reduction of size and weight of the magnetic circuit. However, such magnet material could not take the place of the ferrite magnet due to a strong market tendency towards the low cost.
For other means to achieve the above first subject through employment of the ferrite magnet, there may be considered an improvement of a magnetron output structure. More specifically, in the magnetron for common use, although the output antenna is arranged in the axial direction of an anode cylinder, it is intended, in the improved structure, to dispose the output antenna in a direction normal or perpendicular to the axis of the anode cylinder. The advantage of the above structure is such that the structural dimension of the magnetron main body with respect to the longitudinal direction of its output antenna can be reduced, whereby compact size of the electronic range on the whole may be achieved.
With respect to the high operating efficiency which is the second subject for the improvement, owing to the fact that this efficiency is to be determined by the product of electronic efficiency, which is a conversion factor for converting kinetic energy of electrons emitted from a cathode into high frequency energy, and circuit efficiency, which is a deriving factor for deriving the high frequency energy produced in a resonance circuit of an anode out of the anode, such improvement of the operating efficiency may be ascribed to enhancement of the electronic efficiency or circuit efficiency.
As means for enhancing the electronic efficiency, there may be considered optimization of a relative design of the cathode portion diameter and the inner diameter of the anode surrounding the cathode portion with respect to the number of resonance cavities of the anode resonance circuit can be considered namely, optimization of an interaction space, uniform distribution of magnetic field within said interaction space, and optimization of the high frequency field into the interaction space, and even more particularly optimization of the high frequency field action with respect to electrons.
Of the above-described considerations, with respect to the former two various studies have been made in which a technique therefor has almost been established. By way of example, for the optimization of the interaction space, the ratio of the cathode radius r.sub.c to the anode inner radius r.sub.a is designed to satisfy the relation represented by ##EQU1## with respect to the number of resonance cavities N in order to achieve a stable .pi. mode oscillation. Meanwhile, with respect to the uniformity of the magnetic field distribution, this is dealt with by devising configurations of pole pieces disposed at opposite ends in the axial direction of the interaction space.
However, concerning the optimization of the high frequency field action with respect to electrons, there has been no technique which clearly refers thereto as a structure for the means, about which, description will be given later.
On the other hand, as means for improving the circuit efficiency, there may be conceived enhancement of a value Q for the resonance circuit, i.e. reduction of loss in the resonance cavities, increase of coupling degree between a load circuit and the resonance circuit, etc.
Normally, the latter means is introduced into the resonance circuit as designed by the former means. Incidentally, noises are increased in proportion to the increase of the coupling degree, and thus, although the practice involves some inconsistency with respect to the requirement for low noise which is another subject for improvement to be described below, the optimum coupling degree has been selected while suppressing the noise within the standard.
With respect to the reduction of noise related to the third subject, most of the means employed are arranged to suppress the generated noise by a filter circuit added thereto, but there have also been considered some means adapted to suppress generation of the noise itself. By way of example, there are proposed means for suppressing noise through suppression of turbulence in the electron movement near the forward end side portions of vanes which form the resonance cavity group, by applying proper cuts in the edge in the longitudinal direction of such vane forward end side portions confronting the cathode portion, and means for suppressing noise through suppression of electron flow intending to flow out in the longitudinal direction of the cathode portion by insulating the pole pieces disposed at the opposite ends in the axial direction of the interaction space, from the anode portion.
For solving the various subjects as described so far, there has conventionally been disclosed an interesting construction, for example, in U.S. Pat. No. 4,310,786. The magnetron structure disclosed in said prior art is mainly characterized in a supporting construction of the cathode portion, in which said cathode portion is arranged to be supported by a pair of pole pieces insulated from the anode portion. As a second feature, a set of strap rings are arranged at the central portion in the axial direction of an anode cylinder within the vanes, while an output antenna is disposed in a direction normal to the axis of the anode cylinder as a third feature. Accordingly, with respect to the various subjects as described earlier, the construction of said prior art intends to achieve compact size and low noise, although the operating efficiency thereof is considered to be lower than that of the conventional magnetron due to a reason which is to be described later.
Regarding the construction of the strap rings which is the second feature of the above described prior art, there are also considered various other constructions, part of which is disclosed, for example, in U.S. Pat. No. 3,553,524. In this prior art, it is stated that the position for disposing the strap rings for stably maintaining the .pi. mode oscillation should preferably be within the vanes as compared with the arrangement of such strap rings at upper and lower edges of vanes as employed in the common magnetrons. Meanwhile, it is shown that the strap ring arranging construction in U.S. Pat. No. 4,310,786 referred to earlier is difficult to be applied to the commonly used magnetrons due to increase of manufacturing cost, but this point may be solved if the techniques disclosed in U.S. Pat. Nos. 4,056,756 and 4,179,639 are adopted.
In connection with the above, it is to be noted that, with respect to the effects in use of such strap rings to be arranged within the vanes, specific effects thereof have not been fully clarified up to the present.